Os Odontoideum

Description

Normal anatomy and Os Odontoideum.

Flexion/extension lateral radiographs showing an os odontoideum (arrows) and atlantoaxial instability.

Name

• Os Odontoideum
• Ossa Odontoidea
• Os Avis

General

• Anatomic variant of the odontoid process of the Axis
• Needs to be differentiated from persistent ossiculum terminale, type 2 odontoid fracture
• Smaller than normal dens, but size varies
• Subluxation and instability are common
• One of the accessory bones of the spine

Etiology

• Initially thought to be a congenital lesion due to failure of ossification center in the dens
• May actually represent an unrecognized dens fracture before age 5 or 6

Subtypes

• Orthotopic: normal position with a wide gap between C2 and os odontoideum
• Dystopic: displaced (AKA os avis)

Associated Conditions

• Morquio syndrome
• Multiple epiphyseal dysplasia

Radiographic Features

• Smooth, well-corticated ossicle at the superior component of a hypoplastic dens
• Around half the size of a normal dens
• Associated with hypertrophied and rounded anterior arch of the atlas

History

• First described by Italian anatomist and neuroscientist Carlo Giacomini (1840-1898)

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Clinical Significance

Pathology

• Atlantoaxial Instability

Surgical Considerations

• May require surgical intervention

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See Also

• Accessory Bones Main
• Radiology Main
• Cervical Spine Anatomy Main

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